Zeolites surface acidic properties

Adsorption of pyridine bases is generally applied to test the surface acid properties of zeolites. Adsorbed pyridine base molecules interact with the surface acid site and the strength of the interaction can be monitored by spectroscopic methods. Komiyama et al. [47] obtained in situ molecular AFM images of well ordered arrays of pyridine and P-picoline on the (010)... 

Most of the published information regarding surface acidity and its relation to catalytic activity has involved zeolites of the faujasite structure as found in zeolites X and Y. A smaller number of investigations of mor-denite have been reported. This discussion will concentrate on studies of these two types of zeolites because their acidic and catalytic properties have been most widely investigated, and because they are both of significant industrial importance. 

Since solid acid catalysts are used extensively in chemical industry, particularly in the petroleum field, a reliable method for measuring the acidity of solids would be extremely useful. The main difficulty to start with is that the activity coefficients for solid species are unknown and thus no thermodynamic acidity function can be properly defined. On the other hand, because the solid by definition is heterogeneous, acidic and basic sites can coexist with variable strength. The surface area available for colorimetric determinations may have widely different acidic properties from the bulk material this is especially true for well-structured solids like zeolites. It is also not possible to establish a true acid-base equilibrium. 

The variations of acidic properties in the surface layers and in the bulk solid catalysts after calcination, reduction, or coking were examined by pyridine Nls XPS [4,7] and by the pyridine infrared adsorption techniques, respectively. This provides a means to compare the changes in the characteristic BrBnsted and Lewis acidity functions after those treatment conditions. First of all, TPD of ammonia revealed that both coked and regenerated samples exhibited much decreased acidity as compared with either calcined or reduced samples before the reaction of n-heptane conversion in either N2 or H2 stream [7]. The adsorption of pyridine may cause further perturbation to the Pt4+ or Pt 2+ species in the zeolite as indicated by the increase in binding energies of Pt3d5/2 electrons, as shown in Table 3 and Figure 4,... 

It was found by Nis XPS studies of pyridine-adsorbed samples that after deactivation the surface acidic function changes in a different manner with the bulk acidity measured by infrared characteristic absorption bands of pyridine adsorbed samples [7], which would suggest different distributions of the acidic properties in the sample catalysts. The effects of additive elements on the overall acidic features of modified zeolite catalysts are dependent on sample pretreatment and/or reaction condition, which will contribute differently to the induced acidity on the surface and in bulk bifunctional properties, as examined by the reaction of n-heptane shown in Figure 1. 

Very striking results on the interactions of molecules with a catalyst have been recently reported in zeolite catalysis because of the well ordered structure of these materials it is worth mentioning the subjects of zeolite design [10] and of acidic properties of metallosilicates [11]. In other areas where polycrystallinic or even amorphous materials arc applied, highly interesting results are now numerously emerging (such as hydrocarbon oxidation on vanadium-based catalysts [12] location of transition metal cations on Si(100) [13] CO molecules on MgO surfaces [14] CH4 and O2 interaction with sodium- and zinc-doped CaO surfaces [15] CO and NO on heavy metal surfaces [16]). An illustration of the computerized visualization of molecular dynamics of Pd clusters on MgO(lOO) and on a three-dimensional trajectory of Ar in Na mordenitc, is the recent publication of Miura et al. [17]. 

At the basis of the application of zeolites in fine chemicals reactions is the rich variety of catalytic functions with which zeolites can be endowed. Bronsted acidity, Lewis acidity and metallic functions are well known from classical bifunctional chemistry but for specific reactions, unusual sites, e.g. Lewis acid Ti4+ centres, have been introduced into zeolites. Moreover, zeolites can acquire more or less weakly basic properties metal complexes can be entrapped in zeolite pores or cavities, and enantioselective reactions have been performed by decorating the zeolite surface with chiral modifiers. 

As described above, immersion calorimetry constitutes a powerful technique for the textural and chemical characterization of porous solids. In the absence of specific adsorbate-adsorbent interactions, heats of immersion can be related to the surface area available for the molecules of the liquid. However, the use of polar molecules or molecules with functional groups produces specific adsorbent-adsorbate interactions related to the surface chemical properties of the solid. An adequate selection of the immersion liquid can be used to study hydrophilicity, acid-base character, etc. Table 2 reports the enthalpies of immersion (J/g) into different lineal and branched hydrocarbons (n-hexane, 2-methyl-pentane and 2,2-dimethyl-butane) for Zn exchanged NaX zeolites. 

Upon adsorption of benzophenone on oxides with strongly acidic properties, the phosphorescence spectrum exhibits a structureless band with a Atnax at about 490 nm in addition to the normal phosphorescence of benzophenone. The A max of the excitation spectrum of this band was observed at approximately 380 nm, and its intensity increased in the order of the aluminosilicate, H-mordenite, and HY zeolite. In the spectrum of HY zeolite containing benzophenone, only one structureless phosphorescence band could be observed. A similar phosphorescence band could be observed for benzophenone dissolved in CHCI3, which also involves dry HCl. We can therefore assign phosphorescence at about 490 nm to the protonated form ofbenzophenone. These findings correspond with studies of the photoluminescence of benzophenone dissolved in various concentrated acidic solutions (277). Consequently, since the presence of a phosphorescence spectrum at about 490 nm with benzophenone adsorbed on the aluminosilicate, H-mordenite, or HY zeolite is associated with the presence of the protonated form of benzophenone, the data indicate the existence of proton-donor centers on these oxides with acid strengths < for benzophenone (about 5.6) (216). On HY zeolite, almost all the adsorbed benzophenone changes into protonated benzophenone. On aluminosilicate surfaces, the relative intensities of the phosphorescence spectra attributed to the protonated and unprotonated forms are approximately the same. 

Oare earth forms of zeolites X and Y type faujasites possess superior catalytic properties for various reactions such as alkylation, isomerization, and cracking (9, 12, 18). Structural studies involving x-ray diffraction and CO chemisorption have been made to locate the positions of the rare earth (11, 14, 16). Hydroxyl groups and their relationship to surface acidity have been studied by infrared spectroscopy, utilizing the adsorption of pyridine and other basic molecules (2, 6, 21, 22, 23). Since much of the previous research has involved measurements on mixed rare earth faujasites, a need existed for a more systematic study of the individual rare earth zeolites, in regard to both structural and catalytic properties. The present investigation deals with the Y, La, Ce, Pr, Sm,... 

Conversion of TIPB in dependence on W content, activation temperature (623 and 773 K) and time on stream (TOS) is shown in Figures 6 and 7. It follows that the impregnation of H-ZSM-5 with heteropolyacid has altered the catalytic properties of the external zeolite surface. Decomposition of TIPB distinctly differs in dependence on the activation temperature. All samples exhibited very low initial activities (TOS =15 min) after activation at 623 K. Activation at 773 K primarily enhanced the initial activity of the zeolite itself (sample OW-ZSM-5) but, nevertheless, activities of the modified samples were also considerably increased, even after 55 min time on stream. Obviously, blockage of acidic zeolite sites is mostly removed by calcination at 773 K, possibly due to aggregation of dispersed heteropolyacid but, additionally, this aggregated heteropolyacid should partly contribute to the TIPB conversion because the activity of the modified samples 2W-ZSM-5 and 4W-ZSM-5 exceeds that of the unmodified ZSM-5. 

The hydrothermal deactivation of Y zeolite containing 0, 4, 7 and 12 wt.% of REO and its effects on catalytic activity, stability and selectivity were investigated. The Y zeolites were hydrothermally deactivated at 788°C in three consecutive cycles of two hours each. The fresh and deactivated zeolites were characterized by measuring Unit Cell Size (UCS) and surface area. The acidic properties were measured by the Temperature Programmed Desorption (TPD) of ammonia and IR-pyridine desorption. In order to correlate structural, textural and acid properties with catalytic behavior, the zeolites were evaluated in the conversion of cyclohexane. The Hydrogen Transfer Index (HTI) measured as a ratio of paraffins to olefins is a parameter of the selectivity. It was found that the REO was incorporated into zeolite structure up to high concentrations modifying to some extent XRD deflection, the acidic properties and the HTI ratio. After deactivation, the acidity and HTI were diminished and the Lewis/Bronsted acid ratio was modified. HTI decreased as REO concentration increased. 

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